/* globals d3 */
'use strict';
// source: http://mbostock.github.io/d3/talk/20111018/collision.html
function collisionTick(nodes) {
    var q = d3.geom.quadtree(nodes),
        i = 0,
        n = nodes.length;

    while (++i < n) {
        q.visit(collide(nodes[i]));
    }
}

function collide(node) {
    var r = node.radius + 16,
        nx1 = node.x - r,
        nx2 = node.x + r,
        ny1 = node.y - r,
        ny2 = node.y + r;
    return function(quad, x1, y1, x2, y2) {
        if (quad.point && (quad.point !== node)) {
            var x = node.x - quad.point.x,
                y = node.y - quad.point.y,
                l = Math.sqrt(x * x + y * y),
                r = node.radius + quad.point.radius;
            if (l < r) {
                l = (l - r) / l * 0.5;
                node.x -= x *= l;
                node.y -= y *= l;
                quad.point.x += x;
                quad.point.y += y;
            }
        }
        return x1 > nx2 || x2 < nx1 || y1 > ny2 || y2 < ny1;
    };
}